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What is no IP domain lookup?

Published in Network Administration 2 mins read

"No ip domain lookup" is a command used in network devices, particularly Cisco routers and switches, to prevent the device from attempting to resolve mistyped or unrecognized commands as domain names via DNS (Domain Name System) queries.

Here's a breakdown:

  • The Problem: When you enter an incorrect command on a router or switch's command-line interface (CLI), the device, by default, interprets the unknown command as a potential hostname and attempts to resolve it to an IP address using DNS. This can lead to delays while the device waits for a DNS server to respond (or timeout). This can be frustrating, especially when troubleshooting or configuring the device quickly.

  • The Solution: The no ip domain lookup command disables this behavior. When this command is enabled, the device will not attempt DNS resolution for unrecognized commands, instead simply returning an "Invalid input detected" or similar error message immediately.

Benefits of using no ip domain lookup:

  • Faster Response: Eliminates the delay caused by unnecessary DNS lookups for invalid commands.
  • Reduced Network Traffic: Prevents the generation of potentially unnecessary DNS queries.
  • Improved CLI Experience: Makes the CLI more responsive and efficient, particularly when mistakes are made.

How to use the command (Cisco IOS):

  1. Enter privileged EXEC mode (enable): enable
  2. Enter global configuration mode: configure terminal
  3. Disable IP domain lookup: no ip domain lookup

Example:

Without no ip domain lookup (default behavior):

Router# shwo ip interfac brief  <- Mistyped command
Translating "shwo"...domain server (255.255.255.255)
% Unrecognized command

With no ip domain lookup configured:

Router# shwo ip interfac brief  <- Mistyped command
% Invalid input detected at '^' marker.

Important Notes:

  • This command only affects the router's behavior when it encounters an unrecognized command entered directly into the CLI.
  • It does not disable DNS resolution for other purposes, such as resolving hostnames used in configuration commands (e.g., ping hostname). To prevent name resolution in other parts of a config, consider setting a null domain name as well.

In summary, no ip domain lookup improves the usability of a network device's CLI by preventing it from wasting time attempting to resolve mistyped commands as hostnames via DNS.

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